Steering nibble is the undesirable rotational vibration experienced by the driver at the steering wheel mainly during straight line driving. In some vehicles steering wheel nibble is the result of the chassis system responding to the tire and wheel force variations which eventually feed back in the form of slight rotations in the steering system. In many vehicles, steering nibble is caused by the presence of a front road wheel imbalance or front tire force variation. This steering wheel vibration occurs at a frequency of one times (1×) the rotational velocities of the front road wheels. The vibrations magnitude is maximized when these frequencies align with the steering/suspension resonant frequency, typically 12 to 20 Hz.
Brake judder is due to a chain of events, at the beginning of which there is unequal wear of the brake disks which leads to thickness variation of the brake disk. This disk thickness variation produces a harmonic modulation of the braking force during braking. The oscillations of the braking force in turn excite different modes of the wheel suspension, the vibrations being transmitted via a kinematic coupling to the steering system and in particular to the steering rod, if they lie in a specific critical frequency range, are in turn transmitted to the steering wheel and excite an oscillation of the steering gear, of the steering column and the steering wheel. For brake judder the vibration occurs at a frequency of one times (1×) or two times (2×) the rotational velocities of the front road wheels.
Steering wheel nibble and brake judder are customer concerns in many production automobiles. Original equipment manufacturers and their suppliers are investigating chassis modifications to address and reduce nibble and judder. However, these modifications often have negative effects on other vehicle characteristics, and are typically expensive to implement.
There is a need to detect and actively control steering nibble and brake judder in an electric steering system without affecting steering feel, with the aim of reducing steering nibble related warranty costs.